This invention relates generally to a method of and apparatus for determining the temperature and other thermodymanic characteristics of an object, and particularly relates to a method of and apparatus for measuring the temperature of a highly reflective moving object.
In the prior art various types of temperature measuring apparatus are known, for example, a color pyrometer which is a device designated to measure the temperature of an object by determining the intensity of its radiation at two different wave length regions. Such a two-color pyrometer is quite satisfactory for determining the temperature of an object provided the object is a black or so called gray body. A gray body as used here is defined as having a radiation intensity over different wave lengths which generally follows Plank's laws except that the intensity of its radiation is less by a fixed amount than the radiation of a black body. However, if the object is not a black body or is a non-gray body and therefore has a radiation intensity distribution over different wavelengths which does not follow Plank's laws, a two-color pyrometer will not, in general, indicate the true temperature of the object.
Another type device used to measure the temperature of an object is a total radiation pyrometer. As the name implies this device measures the total radiation emitted by an object. Conventionally, a total radiation pyrometer makes use of a black body which is used as a reference body. In one type of total radiation pyrometer the reference body is held at a fixed temperature and the radiation intensity of the reference body and that of the object being measured are compared after taking into account any differences in the distances between the reference body and its detector and the object and its detector. Again, the object is assumed to have a radiation distribution of either a black body or a gray body.
Another type of total radiation pyrometer has been devised to overcome the disadvantages of the two devices previously discussed. This is also a total radiation pyrometer but has been adapted so that the reference body and an opaque object are positioned in close proximity. As a result, when the temperature of the object and the black body are the same the body is in a substantially isothermal enclosure, i.e. the object is essentially in a black body environment. This construction is advantageous because it ensures that Kirchoff's law is obeyed by the radiation emitted and reflected by the object. When the reference body is not at the same temperature as the object, the sum of the energies emitted and reflected by the object are not numerically equal to that from the black body at the same temperature which is positioned in place of the object.
The radiation emitted by the black body is measured by a first detector and a second detector measures the radiation emitted by the object as well as the radiation of the black body reflected by the object together. It is essential for the operation of such a pyrometer that a black body environment be provided for the object which means that the black body must be in close proximity to the object. Under certain conditions this constraint cannot be met, for example, if the object, the temperature of which is being measured, is located in an induction furnace, it may be impossible to position the black body also in the induction furnace without disturbing the electric field created in the induction furnace and without adversely affecting the operation of the black body. Also, there are definite limits which define the geometric relationship between the object, the black body and the respective detectors. In such a total radiation pyrometer the black body must be of a relatively large size and therefore has considerable thermal lag.
Another apparatus for measuring temperature at the surface of an object using infrared radiation is described in U.S. Pat. No. 3,924,469 which discloses an apparatus comprising a variably heated metallic body which serves as a compensating radiator, a reflective member mounted on a shaft within a cavity provided within the compensating radiator body for rotation, or alternatively oscillation, about an axis forming an oblique angle to the plane of the reflective member such that in one position of the reflective member only infrared radiation from the surface of the object is reflected by it into a radiation detector, while in another position of the reflective member only infrared radiation from a wall surface of the cavity within the compensating radiator is reflected by it into the radiation detector whereby infrared radiation from the object and compensating radiator are admitted to the radiation detector in alternation. The detector then produces an alternating current signal determined by any temperature differential existing between the object and the heat supply to the compensating radiator is varied in accordance with the signal in such sense as to reduce the signal to zero whereby the temperature of the compensating radiator then equals the temperature of the object.
With the exception of attaching dyestuff, dielectric media and other foreign substances to the object and observing their temperature related characteristics from a distance, no practical radiation pyrometry methods of measuring the temperature of an object without physical contact exist. Radiation pyrometry methods have in the past been of limited usefulness because both the radiation being emitted by the object and the radiation being reflected by it influence the value obtained by such methods. In order to ensure precise temperature measurements by use of radiation pyrometry methods it is important to reduce the radiation from extraneous sources which is reflected from the object and extracted by a sensor. Additional radiation pyrometry methods are disclosed in U.S. Pat. Nos. 3,057,200; 3,364,066; 3,413,474; 3,462,602; 3,073,122; 4,172,383; 4,233,512 and general radiation theory is also discussed in standard texts for the study of physics, however, it is not felt that these references are particularly relevant to the invention disclosed and claimed herein.
In the art of continuous casting and rolling of aluminum the difficulties in continuously measuring the temperature of the cast bar between the casting machine and the rolling mill are compounded by the reflective and emissivity characteristics of the cast alulminum bar. The radiative surface properties of a cast aluminum bar are a function of the surface quality and alloy composition surface properties can also be functions of the solidification process itself, for example, inverse segregation will significantly alter the surface characteristics of a cast bar. Also, the amount of thermal radiative energy from the cast bar which reaches an infrared sensor is affected by the intervening atmosphere which absorbs, reflects, scatters and re-emits radiative energy, as well as the geometry of the sensor location relative to the cast bar. The problem of accurately measuring the cast bar temperature of an aluminum cast bar by infrared radiation pyrometry is made even more complex because of the temperature range being measured and because aluminum characteristically has a low and extremely variable (0.02 to 0.6) emissivity and the combination of these factors create problems in discriminating between the signal from the cast bar and extraneous signals from the surroundings.
The exact composition of an aluminum cast bar depends upon the alloy specified by a customer and by specified physical properties which are desired for a finished product to be produced from the bar being cast. All such variations are reflected in changes in both the optical and thermal radiative properties of the bar surface. Also, the amount of surface oxidation, surface scale and other surface characteristics are variable from one alloy to another. Additionally the surface characteristics and indirectly the thermal radiative properties of the cast bar can be changed by variations in process parameters.